Reduced particle contamination manufacturing and packaging for reticles

ABSTRACT

A method of transporting a reticle is disclosed. The reticle is placed in a reticle carrier that has an ionizer. Moreover, the reticle may be attached with a pellicle. The pellicle consists of a pellicle frame and a pellicle film stretched over the pellicle frame. The pellicle frame has included within an absorbent material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority from, U.S.patent application Ser. No. 09/737,125, filed Dec. 13, 2000 now U.S.Pat. No. 6,715,495, which is a divisional of, and claims priority from,U.S. patent application Ser. No. 09/477,795, filed Dec. 30, 1999, U.S.Pat. No. 6,279,249.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photolithography of semiconductordevices, and more particularly, to a method of making and packagingreticles resulting in reduced particle contamination.

2. Background Information

Photolithography is a process that is commonly used in the manufactureof integrated circuits. The process involves the deposition of aphotoresist layer onto an underlying substrate layer. The photoresist isthen selectively exposed to light, which chemically alters thephotoresist. The photoresist is then developed and those portions of thephotoresist that are exposed to light are either hardened or softened,depending upon whether or not the photoresist is negative or positivephotoresist, respectively.

The pattern that is projected onto the photoresist layer is contained ona mask that is placed within the photolithography exposure tool. A mask,also referred to as a reticle, is placed between the illuminating lightand the photoresist. The reticle is typically formed from patternedchromium placed on glass or quartz. The pattern is transferred onto thephotoresist by projecting an image of the reticle onto the photoresistusing an exposing radiation.

In many applications, the reticle is covered by a pellicle. A pellicleis a thin film of optical grade polymer that is stretched on a frame andsecured to the reticle. The pellicle's purpose is to prevent airbornedirt from collecting on the mask and acting as an opaque spot. Duringthe exposure, the dirt is held out of the focal plane and does not printon the wafer.

With the need for smaller critical dimensions, photolithographytechnology has evolved into using extreme ultraviolet (EUV) exposureradiation that has a smaller wavelength. One wavelength that is becomingpopular is 157 nm. Unfortunately, current pellicles are formed from amaterial that are either not sufficiently transparent to radiation at157 nm or does not have sufficient durability under these processconditions. Therefore, in many applications, reticles are manufacturedwithout pellicles. This causes increased risk to contamination of thereticle.

Moreover, after the reticle has been manufactured, the reticle must betransported from the manufacturer to the semiconductor fabricationfacility (known as a “fab”). This transport process increases the riskof contamination. Currently, reticles are stored and shipped in reticlecontainers without any devices specifically designed to remove particlesfrom the environment in which they are enclosed.

SUMMARY OF THE INVENTION

A method of transporting a reticle is disclosed. The reticle is placedin a reticle carrier that has an ionizer.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be described in conjunction with thefollowing Figures, wherein:

FIG. 1 is a schematic diagram illustrating a pellicle carrier formed inaccordance with the present invention;

FIG. 2 shows a flow diagram illustrating the steps of handling a reticlein accordance with the present invention;

FIG. 3 shows a flow diagram illustrating the steps of mounting apellicle onto a reticle in accordance with the present invention;

FIGS. 4 a-c show schematic diagrams of a pellicle formed in accordancewith the present invention;

FIG. 5 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with the presentinvention;

FIG. 6 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with an alternativeembodiment of the present invention;

FIG. 7 shows a flow diagram illustrating the steps of modifying areticle and pellicle combination in accordance with another alternativeembodiment of the present invention; and

FIG. 8 is a schematic diagram of a pellicle frame used with the methodof FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The present invention offers a way to purify the environment in areticle carrier to prevent contamination. In accordance with the presentinvention, the reticle carrier includes an ionizer that produces ions.Because particles and other contaminants often hold a net electricalcharge allowing electrostatic forces to attract and hold particles tothe surface of the reticle, the ions act to neutralize the contaminantswithin the reticle carrier. This prevents the contaminants from adheringto reticle surfaces, and thus mitigating the need for a pellicle.

Specifically, as a means to prevent particles from collecting onpellicle-less mask surfaces, the mask is cleaned and inspected forpattern fidelity, hard defects, and soft defects, including particlesand contamination. The mask is placed in a super clean mini-environmentpod, such as a standard mechanical interface (SMIF) pod. The SMIF pod isa self contained, portable mini-environment that surrounds semiconductorproduction wafers and minimizes exposure to contaminants that couldadversely affect chip performance and yield. The above steps are allconventionally performed in the prior art.

However, in accordance with the present invention, the SMIF pod ismodified to include an ionizer that generates ions for neutralizingcharged contaminants. Specifically, turning to FIG. 1, a SMIF pod 101 isshown. The SMIF pod 101 includes an ionizer 103, a pair of oppositelycharged particle collectors 105, and a reticle 107. The ionizer 103generates ions that will neutralize contaminants within the SMIF pod101. Moreover, the particle collectors 105 are oppositely charged. Thisresults in the contaminants being attracted to one or the other of theparticle collectors 105. Note that in FIG. 1, the reticle 107 ispreferably placed with the chromium side 109 down to prevent particlesfrom landing on the particle sensitive side of the reticle.

The entire process of handling the manufactured reticle is shown in FIG.2. First, at box 201, the reticle is placed faced down into the SMIF pod101. Next, at box 203, the SMIF pod 101 is subsequently placed in atransport box while still inside the clean room. The transport box isthen shipped to outside wafer fabs. Upon arrival at the wafer fab, atbox 205, the transport box is wiped down per standard cleaning protocolsand moved to inside the clean room. At box 207, the mini-environmentSMIF pod 101 is removed from the transport box and either moved to thelithography tools if needed immediately for wafer production or intostorage if not.

When the reticle is required, at box 209, the SMIF pod 101 is interfaceddirectly with the lithography tool so that the reticle is never exposeddirectly to factory ambient air. Automatic transfer and loads of thereticle is done into the photolithography stepper. Finally, at box 211,in situ laser cleaning is performed prior to wafer exposure to removeadsorbed substances. The light source may be either the one used toexpose the wafers or a separate source.

In accordance with the other aspects of the present invention, a methodis provided for removing transmission inhibiting chemical species duringthe manufacturing process of the reticle. Typically, opticalcontaminants exist adsorbed on the reticle and pellicle surfaces, in gasphase in the space between the reticle and the pellicle, or both. In aconventional reticle manufacturing process, a pellicle is mounted on thepatterned side of the reticle to protect it from contamination. Thismounting process is invariably performed in a clean room environment toprevent particles from being introduced into the process.

Because the mounting process is performed in atmospheric air, the gasspace between the pellicle and reticle (hereinafter referred to as the“interpellicle space”) will contain air. Common atmospheric substancesinclude carbon and oxygen containing species such as O₂, CO₂, and H₂O,which are known to inhibit the transmission of radiation havingwavelengths commonly used in the photolithography process, such as 157nm. These species may exist: (1) in the gas phase in the interpelliclespace, (2) adsorbed on the reticle surface, including either thesubstrate or thin film surfaces, (3) adsorbed on and absorbed in thepellicle, (4) or adsorbed on the pellicle frame surfaces.

The transmission of 157 nm wavelength radiation can be reduced by thepresence of even monolayer quantities of such chemical species. Removalof these species or substitution of these species with other materialswhich do not absorb the exposing radiation wavelength is thereforerequired for photolithographic processes, especially those in theextreme ultra-violet region and below.

Therefore, what is provided is a method for mounting a pellicle onto areticle under an optically inert gas environment. The completed reticlecan then be transferred to a photolithography tool. The method describedherein is not dependant upon the gas environment used and therefore canbe applied with a variety of substances. Preferably, the gas environmentshould be optically inert to the exposing radiation wavelength.

In accordance with the present invention, referring to FIG. 3, after thereticle has been patterned, cleaned, and its quality verified tospecification requirements, the reticle (referred to as a mask in FIG.3) is transferred into a process chamber with a pellicle at step 301.The chamber is evacuated and maintained at vacuum or filled with anoptically inert gas (such as nitrogen) at approximately atmosphericpressure (plus or minus 50 psi). By optically inert gas, it is meant agas that has minimal or negligible effect on the transmission ofradiation at the wavelength of the exposure radiation used in thephotolithography stepper. Next, at step 303, the mask and pelliclesurfaces are then exposed to either (1) UV radiation, (2) plasma, (3)ozone, (4) heat, or (5) a combination of the above. This exposure willremove adsorbed and absorbed species. In other words, the treatment willdrive desorption. If an optically inert gas is used rather than vacuum,it is flowed during this process to carry away desorbed substances. Theoptically inert gas, such as nitrogen, may also be heated to acceleratethe desorbtion process.

The time of exposure at step 303 will depend upon the final exposureradiation transmission level required, the exposure option used, thepellicle material chosen, the gas flow rate or vacuum pressure, theconvective mass transfer coefficient (if gas is used), and the speciesadsorbed and absorbed on the incoming photo mask. When the targetedexposure time is reached, the treatment of step 303 (of the UVradiation, plasma, ozone, heat, etc.) is discontinued. The gas flow orvacuum maybe simultaneously discontinued or allowed to continue for aperiod of time. The exact operating process and its optimization woulddepend on the equipment geometry, materials, flow path and other designperimeters.

Next, at step 305, once the target exposure time is reached, the chamberis filled with an optically inert gas (if vacuum was used in steps 301and 303) or the gas flow is stopped (if optically inert gas was used).Next, at step 307, robotic systems mount the pellicle either in situ orafter automated transfer to another station (all under an opticallyinert gas environment).

The assembled reticle is moved to a particle inspection tool eitherthrough a mini-environment pod (such as a SMIF pod) or through a robotictransfer system. In either case, the reticle is maintained in anoptically inert environment. As an option, the particle inspection stepcan be skipped, provided that the risk of particle contamination issufficiently low.

At step 309, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 311, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the pod to the tool and serve as an interfacewhich maintains the optically inert gas environment surrounding thephoto mask.

As an additional protection against transmission inhibiting compounds,polar and hydrogen bonding absorbance can be embedded in the pellicleframe to act as a scavenger for any traces of unwanted gases. Turning toFIGS. 4 a-c, a design for a pellicle along these lines is shown.

As seen in FIGS. 4 a-c, the pellicle 401 includes a pellicle frame 405and a pellicle film 403. The pellicle film 403 is stretched over thepellicle frame 405. The frame is filled with an absorbent material. Theabsorbent material may be one of the well known polar and hydrogenbonding absorbent materials. The absorbent may be present in one or allsides of the pellicle frame. The absorbent acts as a scavenger for anytraces of unwanted gases. Note that the shape and packing density shownin FIG. 4 c is only for illustrative purposes. Note that the interior orexterior frame wall may be constructed or coated with a solid adsorbentmaterial as well.

The above description provides an improved method for manufacturing andattaching the pellicle to the reticle. For some applications, thereticle has already been attached to a pellicle. Even for thesesituations, the present invention provides a method for purifying these“pre-made” pellicle/reticle combinations. By purifying, it is meant theremoval of the interpellicle air and substitution with an opticallyinert gas.

There will be described three different methods for substituting the airin the interpellicle space with an optically inert gas. The methodsdescribed here are not dependant on the gas used and therefore can beapplied with a variety of substances at the users discretion.

Referring to FIG. 5, the first method is referred to as the “vacuumassisted removal” technique. After the reticle has been attached to thepellicle and all of the manufacturing and inspection steps have beencompleted, at step 501, the reticle (attached with a pellicle andreferred to as “pelliclized”) is transferred to a process chamber thathas been evacuated. Evacuation of the interpellicle gas during vacuumpump down relies on a pressure release valve (PRV) as a fluid conduit.The PRV is normally integrated into current pellicle frames to equalizeambient pressure with the pressure in the interpellicle space. The size,number, and placement of PRV's may be changed from existingconfigurations to accommodate this new evacuation function. The PRV maybe as simple as a single small orifice in the frame.

After a suitable vacuum has been achieved, at step 503, the pelliclizedreticle is exposed to either ultra violet radiation, plasma, ozone,heat, or a combination of the above to remove the adsorbed and absorbedspecies. The vacuum is augmented with UV or these other methods to drivedesorbtion and removal of contaminants. The time of exposure will dependon the final transmission level required and other factors noted above.This step 503 is similar to step 303 described above.

Next, at step 505, once the target exposure time is reached, the chamberis filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. The rate of the gas fill will bedependent on the rate that the PRV devices allow gas to enter theinterpellicle space. Next, at step 507, the pelliclized reticle is movedinto a mini-environment pod (such as a SMIF pod) through a robotictransfer system. In either case, the reticle is maintained in anoptically inert environment.

At step 509, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 509, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

In the second embodiment referred to as “passive removal” and shown inFIG. 6, after the reticle has been pelliclized and completed all of itsmanufacturing and inspection steps at step 601, it is transferred to aprocess chamber at step 603 where it is exposed to a continuous flow ofoptically inert gas and either UV radiation, plasma, ozone, heat, or acombination of the above. The flowing gas creates a chemical potentialgradiant of the transmission inhibiting compounds across the reticle/gasphase interface to drive mass transfer. The chemical potential gradiantis augmented with ultra-violet or other means to drive desorbtion andremoval of contaminants. To accelerate the desorbtion kinetics, the gasalso may be heated to a higher temperature prior to chamberintroduction. Like the above methods, the time of exposure will dependon various factors discussed above.

Next, at step 605, once the target exposure time is reached, the chamberis filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. Next, at step 607, the pelliclizedreticle is moved into a mini-environment pod (such as a SMIF pod)through a robotic transfer system. In either case, the reticle ismaintained in an optically inert environment.

At step 609, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 609, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

Finally, in the third method referred to as “convective removal” andshown in FIG. 7, after the reticle has been pelliclized at step 701 andcompleted all of its manufacturing and inspection steps, at step 703, itis transferred to a chamber where optically inert gas is routed throughthe interpellicle space through a frame which has been modified toreceive and distribute gas while the reticle exterior is exposed to acontinuous flow of gas. Additionally, either UV radiation, plasma,ozone, heat or a combination of the above is used to remove the adsorbedand absorbed species. The inert gas flows are augmented with UV or othermeans to drive desorbtion and remove all contaminants. Once again, thetime of exposure will tend to depend upon the final transmission levelrequired.

Turning to FIG. 8, a pellicle frame 801 formed in accordance with thepresent invention is shown. The pellicle frame 801 includes a manifoldthat connects a gas source 803 to the interpellicle space. The pellicleframe 801 is equipped with a valve device 805 for flow modulation andshutoff as well as a filter 807 for particulate control. The valve 805and filter 807 replaces the traditional PRV devices.

Next, at step 705, once the target exposure time is reached, the chamberis filled with an optically inert gas, such as nitrogen, untilatmospheric pressure is reached. Next, at step 707, the pelliclizedreticle is moved into a mini-environment pod (such as a SMIF pod)through a robotic transfer system. In either case, the reticle ismaintained in an optically inert environment.

At step 709, the reticle is maintained in an optically inert gasatmosphere during subsequent storage and shipping by means ofmini-environment SMIF pods. As seen in step 709, loading ontophotolithographic tools can be achieved by use of robotics systems whichtransfer the reticle from the SMIF pod to the tool and serve as aninterface which maintains the optically inert gas environmentsurrounding the reticle.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A method, comprising: inserting a pellicle and a reticle into aprocess chamber, wherein the pellicle is not mounted to the reticle;exposing the un-mounted pellicle and the reticle, while in the processchamber, to a treatment for desorption of optical contaminants that maybe adsorbed on or absorbed in the pellicle and the reticle; introducingan optically inert gas into said process chamber; and mounting thepellicle onto the reticle in the process chamber.
 2. The method of claim1 wherein the treatment comprises exposure to at least one ofultraviolet radiation, ozone, heat, and plasma.
 3. The method of claim1, further comprising evacuating the process chamber.
 4. The method ofclaim 1 wherein during the treatment, the process chamber is in a vacuumstate.
 5. The method of claim 1, further comprising flowing theoptically inert gas through the process chamber during the treatment. 6.The method of claim 5, further comprising heating the optically inertgas.
 7. The method of claim 1 wherein the optically inert gas isnitrogen.
 8. The method of claim 1 wherein the pellicle is mounted ontothe reticle in an optically inert gas environment.
 9. The method ofclaim 1, further comprising placing the pellicle mounted onto thereticle in a standard mechanical interface (SMIF) pod.
 10. The method ofclaim 1 wherein the pellicle includes a pellicle film stretched over apellicle frame.
 11. The method of claim 10 wherein the pellicle frameincludes a manifold formed within the pellicle frame.